1. Field of the Invention
The present invention relates generally to syringes for injectors, and more particularly to such syringes which distend under pressure, actuating a sensor.
2. Description of the Prior Art
In the medical field, prior art injectors are known (U.S. Pat. No. 4,465,475) which permits long-term infusions by the slow and continuous squeezing of a syringe. The syringe is fixed by its cylinder to a holder of the injector. A slide pushes the piston rod of the syringe forward so that the infusion solution contained in the syringe is steadily discharged from the cylinder at a predetermined selectable rate. When an obstruction occurs in the fluid line that leads from the syringe outlet to the patient, or when the piston reaches its forwardmost position and strikes against the end of the cylinder, an alarm is triggered which indicates to the attendants that the infusion has been disturbed, or that the cylinder has been emptied and may require replacement. The holder to which the cylinder is attached is designed to move longitudinally. This motion is inhibited by a spring. When the fluid line leading from the cylinder to the patient is obstructed, or when the piston strikes the end of the cylinder, the holder is displaced by the force exerted upon the cylinder, counter to the action of the spring, whereby a micro-switch is actuated which triggers the alarm.
This apparatus of the prior art entails certain disadvantages. The mechanism described above for sensing pressure in the cylinder is relatively complicated, requiring that the cylinder holder be moveably attached to the injector. Further, friction between the piston and cylinder affects the triggering point of the alarm. Since this friction may vary from one syringe to another, there is no assurance that the alarm will be triggered by the same fluid pressure in a cylinder.
If excessive pressure in the cylinder were determined by measuring the current drain of the injector motor which operates the syringe piston, variations in syringes would again engender inaccuracies.